Perdoname que no hablo bien el español. (Estoy apreniendo.)
If the WRF translation of avalan meaning to guarantee or support is correct, then that may not quite be the equivalent of the English.
Attest is related to the term for bear witness, to "testify", and it means the person is officially verifying what is signed. For example, an "attesting witness" is "a person who signs his name to an instrument...in order to prove or identify it." (Cochran's Law Lexicon, 5th Ed.) See also here. Wait for other English-speaking attorneys with better Spanish than mine to comment, however.